1. Field of the Invention
The present invention relates to an inspection apparatus using nuclear-magnetic-resonance (NMR), or an NMR inspection apparatus, which enables an examiner to designate directly a section of a subject to be imaged while watching the subject.
2. Description of Related Art
The nuclear magnetic resonance-imaging apparatus, or MRI apparatus, is a medical image-processing apparatus that causes the nuclear magnetic resonance of water molecules in a section of a subject and reconstructs a sectional image from NMR signals. In general, a slicing magnetic field gradient is applied to a region of a subject to be examined to designate a section to be imaged in the subject. At the same time, an excitation pulse is applied to the subject to excite its spins. An MRI signal (echo) is generated and measured in the phase of focusing of the excited spins. To add positional information to the excited spins, a phase-encoding magnetic field gradient and a read-out magnetic field gradient normal to each other are applied to the subject for the time period from the excitement of the subject to the acquisition of the echo. Measured echoes are arranged in the k-space by an axis of abscissa xe2x80x9ckxxe2x80x9d and an axis of ordinate xe2x80x9ckyxe2x80x9d. Each echo is represented by a line parallel to the kx-axis. The data in the k-space undergo inverse Fourier transformation to reconstruct an image of the designated section.
The pulse to generate an echo and the magnetic field gradients are applied to the subject in accordance with prescribed pulse sequences. Various pulse sequences for various purposes are known. For example, in the gradient-echo method, or GE method, the pulse to generate an echo is repeatedly applied to a subject. Every time the pulse is applied to the subject, the phase-encoding magnetic field gradient applied to the subject is changed. In this way, the necessary number of echoes for reconstructing a sectional image are measured.
FIG. 1A shows pulse sequences for the GE method. A pulse 201 for a slicing magnetic field gradient in the z-direction and an RF pulse 202 of proton""s resonance frequency xe2x80x9cf0xe2x80x9d for a high-frequency magnetic field to excite the spins of a subject to be examined are applied to the subject in order to induce the NMR of protons in the subject. Then, a pulse 203 for a phase-encoding magnetic field gradient is applied to the subject to add positional information in the phase-encoding direction (y-direction) to the phase of the excited spins and another pulse 206 for a read-out magnetic field gradient for dephasing in the x-direction is applied to the subject. Next, while a pulse 207 for a read-out magnetic field gradient is being applied to add the positional information in the read-out direction (x-direction) to the excited spins, an NMR signal (an echo) 208 is measured. The necessary number of echoes for reconstructing a sectional image are measured while the procedure from the application of the pulse 201 to the measurement of the echo 208 is being repeated in a xe2x80x9cTRxe2x80x9d cycle. The echoes are arranged in the k-space (209) as shown in FIG. 1B and undergo two-dimensional inverse Fourier transformation to reconstruct a sectional image.
In MRI, while watching an image displayed on a display, the examiner designates a section to be imaged as follows. First, an image of the whole region of interest is taken as a reference image and displayed on the display. Second, a line segment is displayed on the display and the position and angle of the line segment is moved and changed by using a mouse or a cursor to designate a section and take its image. As the need arises, the examiner designates and images a section repeatedly while making changes on the reference image. There is a method of designating a section of a subject by using a plate-like joystick instead of a mouse (JP-A No. 238915/1997: prior art-1). This method enables the examiner to designate a section intuitively because he can designate the inclination of a section as the inclination of the joystick.
In accordance with the prior art-1, the examiner, watching an image on a display at a distance from the subject, designates indirectly a section to be imaged; accordingly, it is difficult for him to grasp the position of a section being designated. In accordance with the prior art-1, the examiner can directly designate the inclination of a section to be imaged with a joystick, but he cannot directly designate the position of the section because the joystick is typically fixed to something.
The present invention provides a NMR inspection apparatus that enables the examiner to designate directly a section of a subject to be imaged.
One aspect of the present invention is directed to an NMR inspection apparatus that comprises (i) means for generating a static magnetic field, (ii) means for generating magnetic field gradients in first, second, and third directions normal to one another, (iii) means for generating a high-frequency magnetic field, (iv) means for detecting NMR signals derived from a subject, (v) means for processing detected NMR signals, (vi) a section-designating unit for designating a plane including a section of the subject to be imaged, and (vii) a control unit for controlling the means for generating magnetic field gradients, the means for generating a high-frequency magnetic field, the means for detecting NMR signals, the means for processing detected NMR signals, and pulse sequences for taking an image of the section of the subject to be imaged. The means for generating magnetic field gradients is controlled so that a prescribed part of a plane designated by the section-designating unit is the section of the subject to be imaged.
The section-designating unit has reference objects to be set at two or three reference points and is held in the hand of an examiner to be moved and operated in the immediate neighborhood of the subject lying in static magnetic field and magnetic field gradients generated by the means for generating a static magnetic field and the means for generating magnetic field gradients. The examiner, watching the subject, can move and operate the section-designating unit as if he were handling the probe of ultrasound diagnosis equipment. In a preferred embodiment of section-designating unit, the examiner can designate a section of the subject to be imaged by putting the unit into contact with the subject. In another preferred embodiment of section-designating unit, the examiner can designate a section of the subject to be imaged without putting the unit into contact with the subject.
By using the NMR signals derived from the two or three reference objects by the pulse sequences for imaging, the means for processing detected NMR signals finds a plane which includes the middle point between the two reference objects and whose normal vector is a vector connecting the two reference objects or a plane including the three reference objects in terms of reference spatial coordinates. The control unit controls the means for generating magnetic field gradients so that a prescribed part of the plane is the section of the subject to be imaged.
Preferred methods of NMR examination in accordance with the present invention are summarized below.
(1) A method of NMR examination comprising the steps of: (i) designating a plane including a section of the subject to be imaged with the section-designating unit, which has reference objects to be set at two or three reference points and is held in the hand of an examiner to be moved and operated in the immediate neighborhood of the subject lying in the static magnetic field and magnetic field gradients, (ii) finding a plane which includes the middle point between the two reference objects and whose normal vector is a vector connecting the two reference objects or a plane including the three reference objects in terms of reference spatial coordinates, and (iii) controlling the magnetic field gradients so that a prescribed part of the plane is the section of the subject to be imaged.
(2) The method of NMR examination of paragraph (1) further comprising the steps of: (i) taking the images of the reference objects projected in the first, second, and third directions by applying a magnetic field gradient in a first direction to the subject after the application of a high-frequency magnetic field to the subject, applying a magnetic field gradient in a second direction to the subject after the application of another high-frequency magnetic field to the subject, and apply a magnetic field gradient in a third direction to the subject after the application of yet another high-frequency magnetic field to the subject, the two or three reference objects being in the shape of a ball and containing a substance other than water and fat, and (ii) locating the centers of the two or three reference objects by using the projected images in the first, second, and third directions.
(3) The method of NMR examination of paragraph (1) wherein the section-designating unit has selector switches to choose appropriate ones from various parameter values stored in the control unit and the control unit controls the pulse sequences for imaging in accordance with the parameter values chosen by the selector switches to take an image of the section of the subject to be imaged.
(4) The method of NMR examination of paragraph (1) wherein the two or three reference objects are light-emitting diodes (hereinafter referred to as xe2x80x9cLEDsxe2x80x9d) flashing on and off in different cycles and there is provided a step of locating the centers of light-emitting faces of the LEDs by using the images of the LEDs taken by TV cameras.
(5) The method of NMR examination of paragraph (1) wherein there are provided a first display means disposed in a room shielded from electromagnetic waves and a second display means disposed outside the room and an image of the section of the subject to be imaged is displayed simultaneously on both the first and second display means.
(6) The method of NMR examination of paragraph (1) wherein (i) there are provided a first display means disposed in a room shielded from electromagnetic waves and a second display means disposed outside the room, (ii) the section-designating unit has an imaging switch for starting to take an image of the section of the subject to be imaged, and (iii) an image of the section of the subject to be imaged is displayed simultaneously on both the first and second display means while the imaging switch is off and an image of a higher resolution is displayed on both the first and second display means when the imaging switch is turned on.
(7) The method of NMR examination of paragraph (1) wherein the section-designating unit is put into contact with the subject to designate a plane including the section of the subject to be imaged.
(8) The method of NMR examination of paragraph (1) wherein the section-designating unit designates a plane including the section of the subject to be imaged without coming into contact with the subject.
The NMR inspection apparatus and the method of examination of the present invention enable the examiner to designate directly a section of a subject to be imaged easily and quickly while watching the subject.
Other and further objects, features and advantages of the invention will appear more fully from the following description.